The removal of nitrogen and phosphorus from reject water of municipal wastewater treatment plant using ferric and nitrate bioreductions

Reject water, which is the liquid fraction produced after dewatering of anaerobically digested activated sludge on the municipal wastewater treatment plants (MWWTPs), contributes up to 80% of the nitrogen and phosphorus loads to the MWWTP. It was proposed to combine the removal of nitrogen from reject water using the sequential biooxidation of NH4+ and bioreduction of NO3- with precipitation of phosphate by Fe2+ ions produced due to bioreduction of Fe3+ in iron ore. Bioreduction of NO3- decreased Fe3+ bioreduction rate in reject water from 37 to 21 mg Fe2+/L d due to competition between NO3- and Fe3+ for electron donors. Addition of acetate as electron donor increased both bioreduction rates of Fe3+ and NO3- but acetate interfered with the competition between nitrate and phosphate anions reacting with ferrous cations decreasing efficiency of the phosphate removal from reject water. The stages of denitrification and ferric bioreduction/phosphate precipitation must be performed sequentially.